Again within the early 1930s, there have been only some identified elementary particles that made up the Universe. Should you divided up the matter and radiation we noticed and interacted with into the smallest doable elements we may break them up into on the time, there have been solely the positively charged atomic nuclei (together with the proton), the electrons that orbited them, and the photon. This accounted for the identified components, however there have been a number of anomalies that didn’t fairly line up.
Heavier components additionally had extra cost, however argon and potassium have been an exception: argon solely had a cost of +18 models, however a mass of ~40 atomic mass models, whereas potassium had a cost of +19 models, however a mass of ~39 models. The 1932 discovery of the neutron took care of that one. Sure varieties of radioactive decay — beta decays — appeared to not preserve vitality and momentum, resulting in Pauli’s 1930 hypothesizing of the neutrino, which wouldn’t be found for an additional 26 years. And the Dirac equation predicted unfavourable vitality states, which corresponded to antimatter counterparts for particles just like the electron: the positron.
Nonetheless, nothing may have ready physicists for the invention of the muon: an unstable particle with the identical cost, however lots of of occasions the mass, of the electron. Right here’s how this shock actually did flip physics on its head.
The story begins approach again in 1912, when adventuresome physicist Victor Hess had the sensible thought to take a particle detector with him on a scorching air balloon flight. You would possibly marvel what the motivation could be for this, and it got here from an unlikely supply: the electroscope (above). An electroscope is simply two skinny items of conducting, metallic foil, linked to a conductor and sealed inside an airless vacuum. Should you cost up the electroscope, both positively or negatively, the like-charged items of foil will repel one another, whereas for those who floor it, it turns into impartial, and goes again to the uncharged place.
However right here was the unusual factor: for those who left the electroscope alone, even in a reasonably excellent vacuum, it nonetheless discharged over time. Irrespective of how good you made your vacuum — even for those who positioned lead shielding round it — the electroscope nonetheless discharged. Furthermore, for those who carried out this experiment at larger and better altitudes, it discharged extra shortly. This was the place Hess acquired his massive thought, imagining that high-energy radiation, with each excessive penetrating energy and of extraterrestrial origin, was the perpetrator.
If there are charged cosmic particles zipping by Earth’s ambiance, they might assist neutralize this cost over time, because the oppositely-charged particles could be drawn to the electrode and the like-charges could be repelled by it. Hess imagined that there was a really actual “zoo” of particles zipping round by house, and that the nearer he acquired to the sting of Earth’s ambiance (i.e., the upper altitudes he went to), the extra probably he’d be to watch these particles straight.
Hess constructed a detection chamber that contained a magnetic subject, in order that any charged particles would curve in its presence. Primarily based on the path and curvature of any particle tracks that appeared within the detector, he may reconstruct what the rate of the particle was in addition to its charge-to-mass ratio. Hess’s earliest efforts instantly paid off, as he started discovering particles in nice abundance, founding the science of cosmic ray astrophysics within the course of.
Many protons and electrons have been seen in these early cosmic rays, and afterward, the primary antimatter particles have been found this fashion as effectively. However the massive shock got here in 1933, when Paul Kunze was working with cosmic rays and located a particle that didn’t fairly match. It had the identical cost as an electron, however was concurrently far too heavy to be an electron whereas additionally being far too gentle to be an antiproton. It was as if there was some new kind of charged particle, of an intermediate mass between the opposite identified particles, that instantly introduced, “hey, shock, I exist!”
The upper in altitude we went, the extra cosmic rays we noticed. On the highest altitudes, the overwhelming majority of cosmic rays have been neutrons and electrons and protons, whereas solely a small fraction of them have been muons. Nevertheless, as detectors acquired an increasing number of delicate, they began to have the ability to detect these cosmic rays at decrease altitudes, even nearer to sea degree. At this time, for about $100 and with off-the-shelf supplies
, you may construct your individual cloud chamber and detect cosmic ray muons — probably the most plentiful cosmic ray particle at sea degree — at dwelling.
Over the subsequent few years, scientists labored exhausting to detect these muons not from high-altitude experiments, however to watch them in a terrestrial laboratory. In principle, they have been being produced by what we name cosmic ray showers: the place particles from house hit the higher ambiance. When this happens, interactions from the fast-moving cosmic particles that strike the stationary atmospheric particles produce numerous new particles-and-antiparticles, with the most typical product being a short-lived, unstable particle often known as a pion.
The charged pions reside just for nanoseconds, decaying into muons, amongst different particles. These muons are additionally short-lived, however a lot longer-lived than the pion. With a imply lifetime of two.2 microseconds, they’re the longest-lived unstable particle aside from the neutron, which has a imply lifetime of round 15 minutes! In principle, not solely ought to these cosmic ray showers produce them, however any collision of particles that had sufficient vitality to supply pions must also yield muons that we may examine in a lab. The muon, in our detectors, look similar to electrons do, besides they’ve 206 occasions the electron’s mass.
In reality, for those who maintain out your hand and level your palm in order that it faces up, in direction of the sky, roughly one muon (or anti-muon) will go by your hand with every second that goes by. At sea degree, 90% of all of the cosmic ray particles reaching Earth’s floor are muons, with neutrons and electrons making up many of the relaxation. Earlier than we had even found mesons, that are composite quark-antiquark mixtures, unique, heavy, unstable baryons (that are mixtures of three quarks, like protons and neutrons), or the quarks that underlie matter, we had found the muon: the heavy, unstable cousin of the electron.
As quickly because the physicist I. I. Rabi, who himself would win the Nobel Prize for the invention of nuclear magnetic resonance (right this moment used ubiquitously in MRI expertise), realized concerning the muon, he famously quipped, “who ordered that?” With so few particles identified on the time, including this unusual cousin of the electron — heavy, unstable, and short-lived — appeared like a phenomenon of nature that defied rationalization.
We have been many years away from uncovering the character of matter and the construction of the Commonplace Mannequin, however the muon was our very first clue that there have been not solely extra particles on the market ready to be found, however that particles got here in a number of generations. The primary-generation of particles are the secure ones, consisting of the up and down quarks, the electron and the electron neutrino, and their antimatter counterparts. At this time, we all know of two extra generations: the second-generation, which has attraction and unusual quarks with muons and muon neutrinos, and the third-generation, which has high and backside quarks with tau and tau neutrino particles, plus their analogous antimatter counterparts.
The muon, nonetheless, didn’t merely foreshadow all of those new discoveries, nevertheless it additionally yielded an thrilling and counterintuitive demonstration of Einstein’s relativity. The muons that get created from cosmic ray collisions, on common, originate at an altitude of 100 kilometers. Nevertheless, the imply lifetime of a muon is just 2.2 microseconds. If a muon moved extraordinarily near the velocity of sunshine at 300,000 km/s, you are able to do slightly math, multiplying that velocity by the muon’s lifetime, to search out that they need to journey about 660 meters earlier than decaying.
However muons arrive at Earth’s floor, journeying 100 kilometers and nonetheless not decaying! How is that this doable? With out relativity, it wouldn’t be. However relativity brings alongside the phenomenon of time dilation, enabling particles that transfer near the velocity of sunshine to expertise time passing extra slowly than they do for observers at relaxation. With out time dilation, we might by no means have found these cosmic muons, and we wouldn’t have the ability to see them in our terrestrial cloud chambers, not except we created them from particle accelerators. Einstein, regardless of not realizing it, helped us uncover this essentially new type of matter.
Wanting forward, with the ability to management and manipulate these muons simply would possibly result in advances in experimental particle physics that no different kind of collider can match. While you construct a particle accelerator, there are solely three elements that decide how energetic your collisions are:
how massive your ring is, with bigger circumference rings reaching larger energies,
how sturdy your magnetic fields that bend your charged particles are, with stronger magnets resulting in larger energies,
and the charge-to-mass ratio of your particle, with low lots resulting in synchrotron radiation and a limiting vitality, and excessive lots not having that drawback.
That third issue is why we use protons as an alternative of electrons in accelerators just like the Massive Hadron Collider at CERN, however there’s a downside: protons are composite particles, and solely a tiny fraction of its complete vitality winds up in a quark or gluon that collides with one other. However the muon doesn’t endure from that downside, and it additionally isn’t restricted by synchrotron radiation like electrons are, as a consequence of its a lot heavier mass. If we are able to grasp muon accelerators, we simply would possibly unlock the subsequent frontier in experimental particle physics.
At this time, we are able to look again on the invention of the muon as quaint, with our scorching air balloons and primitive detectors revealing these uniquely bent particle tracks. However the muon itself continues to offer a legacy of scientific discoveries. From its energy in illustrating the results of time dilation on a particle’s noticed lifetime to its potential to result in a essentially new, superior kind of particle accelerator, the muon is a complete lot extra than simply background noise in a few of our most delicate, underground experiments looking for the rarest particle interactions of all. Even right this moment, the experiment to measure the muon’s magnetic dipole second might be the important thing that takes us, ultimately, into understanding physics past the Commonplace Mannequin.
Nonetheless, when it unexpectedly introduced its existence within the 1930s, it was really a shock. For all of historical past earlier than then, nobody had imagined that nature would make a number of copies of the basic particles that underpinned our actuality, and that these particles would all be unstable towards decays. The muon simply occurs to be the primary, lightest, and longest-lived of all of these particles. While you consider the muon, bear in mind it as the primary “technology 2” particle ever found, and the primary clue we ever acquired as to the true nature of the Commonplace Mannequin.